This invention relates generally to the fields of catalytic and pyridine chemistry, and in particular, to an improved heterogeneous-supported catalyst and process for the carbonylation of methanol in the production of acetic acid.
From a historical viewpoint, carbonylation and related processes for the preparation of carboxylic acids and esters from corresponding alcohols or their derivatives are long and well known in the art. This is particularly true of the carbonylation of methanol in the production of acetic acid, which has enjoyed a significant worldwide market for many years.
As general background, reference can be made to Kirk-Othmer, Encyclopedia of Chemical Technology 1, 135 et seq. (1978), which highlights this history as well as the major contribution by the Monsanto Company in the early 1970's in developing a process for methanol carbonylation based on an iodide-promoted catalyst incorporating rhodium metal complexed with phosphines. This catalyst was homogeneous, and therefore soluble in the reaction medium requiring its recovery from the still bottoms following carbonylation so that recycling of the expensive rhodium metal could be effected. This Monsanto process, as it is known and commonly referred to in the industry, has received widespread acceptance to the extent that the great majority of the world's production of acetic acid is now accomplished by its basic teaching. Considerable publications have appeared over the years describing and analyzing this Monsanto process, including its facets and benefits. The leading U.S. patent for the process appears to be U.S. Pat. No. 3,769,329 issued to Paulik et al. in 1973, which is hereby incorporated and hereby referenced to the extent that any further explanation or understanding of this Monsanto process is required for the purposes of the present application.
There has been limited attention given to this Monsanto process for acetic acid synthesis with regard to possible supported versions of its homogeneous catalyst. Several laboratories have attempted ionic attachments to both organic and inorganic polymers, but with limited success. Although supported catalysts having reactivities approaching their homogeneous analogs have been reported, deactivation of the rhodium by the polymeric support or other difficulties have been the norm rather than the exception. This is somewhat expected from the common experience and thinking in the industry that some deactivation will result when a heterogenous support is employed.
As examples of this work, some of the earliest attempts were to place a rhodium compound on carbon or alumina for use in a vapor phase reaction. In this regard, Jarrell and Gates, J. Catal., 40, 255 (1975) used a standard kind of phosphine-containing polymer to support rhodium for both liquid and gas phase reactions. They reported that the catalyst lost activity rapidly due to rhodium leaching from the support even at exceptionally low temperatures in the range of 85.degree.-95.degree. C. Further attempts are reviewed, for example, in Forster, Adv. Organomet. Chem., 17, 255-267 (1979) and in Scurrell, Platinum Metals Review, 21(3), 92-96 (1977).
More recently, another article has described the possibility of supporting a rhodium compound on an ionic resin such as Dowex 1-X8, Bio-Rex 9, or a possible copolymer of styrene and 4-vinylpyridine alkylated with methyl iodide. Drago et al., Inorg. Chem., 20, 641-644 (1981). This article included no experimental results using the suggested polyvinylpyridine derivative catalyst. From the examples that were given, the authors concluded that their ionically-supported rhodium catalyst was approximately equal in catalytic activity to the homogeneous complex, and that leaching of the catalyst could be minimized by suitable choice of solvent and by selecting high resin:rhodium ratios. In all tests reported, however, only low temperatures of 120.degree. C. and low pressures of 80 psi were used. Moreover, it was believed that doubling the amount of supported catalyst (and thus the rhodium present) resulted in a corresponding doubling of the reaction rate, and that an effective method for carrying out the reaction may be to maintain large concentrations of catalyst particularly in a liquid-flow system design.
In a later-issued patent, U.S. Pat. No. 4,328,125, Drago et al. similarly used only mild temperatures at about 120.degree.-130.degree. C. and pressures ranging from less than 60 psi to 160 psi in one example. These conditions, and particularly the low temperatures, are wholly impractical for any commercial use in acetic acid production, and are specifically far outside the Monsanto process conditions of temperatures at about 170.degree.-200.degree. C. and pressures at about 65-80 Bar. As a result, the reaction rates are so low as compared to the homogeneous process that large reactions with increased material costs, long residence times, and resulting low space-time yields would be needed to have any hope of producing a commercial product. These mild Drago et al. conditions are nonetheless required by most ion exchange resins which are not stable, for example, at elevated temperatures above 170.degree. C. and approaching 200.degree. C. The patent concludes that large concentrations of catalysts relative to liquid, particularly in a flow process, will result in very rapid reaction rates with the process preferrably being carried out at these lower temperatures under less corrosive conditions than processes using conventional homogeneous catalysts. Although polyvinylpyridines, as such, are mentioned at one point in the Drago et al. patent, no examples are given of their preparation or use. The examples are instead limited to one example of a polystyrene bound pyridine and to commercially available anionic exchange resins identified as Amberlite lRA-400 and Dowex 1-X8 which are used in applications as catalysts for the hydroformulation of olefins.
Importantly, there is no disclosure or suggestion in this Drago et al. article and Patent, or in any other reference known, evidencing an appreciation of any significance of the degree or amount of metal loading of the catalyst as it relates directly to reaction performance, nor is there any teaching, disclosure, or suggestion in any reference known to Applicants evidencing an appreciation of any significance of a particular type, class or characteristic of polymer support as it relates directly to reaction performance. The only beliefs of this type known to have been reported, as already mentioned for Drago et al., are in some direct dependence of reaction rate on increasing catalyst concentrations or amount and in some possible effect on leaching by increasing the resin:rhodium ratio.